1. Field of the Invention
The present invention relates to a liquid crystal display device which uses a nematic crystal composition having positive dielectric anisotropy (Δ∈>0).
2. Description of the Related Art
Currently, regarding the devices of the active matrix drive system, display modes such as an optically compensated bend (OCB) mode, a vertical alignment (VA) mode and an in-plane switching (IPS) mode have been applied, due to their display quality, to portable terminals, liquid crystal TV sets, projectors, computers, and the like. Since an active matrix display system has a non-linear circuit provided for each pixel, and it has been suggested to use a thin film transistor (TFT) using amorphous silicon or polysilicon, or an organic semiconductor material. Furthermore, as a method for the alignment of liquid crystal molecules to cope with an increase in display size or high definition display, it has been suggested to use a photo-alignment technology. It has been suggested to use a phase difference film in order to obtain wider viewing angle characteristics for the display, or to use a photopolymerizable monomer in order to obtain clear display (SID Sym. Digest, 277 (1993); SID Sym. Digest, 845 (1997); SID Sym. Digest, 1077 (1998); SID Sym. Digest, 461 (1997); Proc. 18th IDRC, 383 (1998); SID Sym. Digest, 1200 (2004); Proc. Asia Display, 577 (1995); and Proc. 18th IDRC, 371 (1998)).
However, in order for liquid crystal display television sets to completely replace the conventional television sets utilizing cathode ray tubes (CRT) and to also cope with the demand for 3D imaging or field sequential display, liquid crystal TVs are still not satisfactory in terms of the response speed and viewing angle characteristics. For example, the IPS mode is excellent in the viewing angle characteristics, but is not satisfactory in terms of the response speed; and the VA mode exhibits a relatively fast response speed, but is not satisfactory in terms of the viewing angle characteristics. Accordingly, in addition to the use of the overdrive mode, an amelioration for enhancing the apparent response speed of display elements by changing the frame frequency from 60 Hz to a high frequency such as 120 Hz or 240 Hz, has been in progress. However, there are limitations in overcoming the limit of the response speed that is intrinsic to a liquid crystal material, if amelioration is made only in terms of the electronic circuit of these liquid crystal display devices. Thus, there is a demand for a drastic improvement in the response speed as a result of amelioration in the entirety of a display device including a liquid crystal material.
Furthermore, in order to improve the viewing angle characteristics in regard to the VA mode, a multi-domain vertical alignment (MVA) mode has been suggested in which the viewing angle characteristics are improved by partitioning the pixels, and changing the direction of orientation of the liquid crystal molecules for each of the partitioned pixels. In this mode, it is possible to improve the viewing angle characteristics; however, since it is required to produce liquid crystal cells that have a complicated structure uniformly in order to achieve pixel partitioning, a decrease in production efficiency has been unavoidable.
As a method of drastically improving such a problem, new drive systems that are different from the conventional drive systems have been suggested. For example, there is known a method of aligning a liquid crystal material having positive dielectric anisotropy (Δ∈>0) perpendicularly to the substrate surface without voltage application, and driving liquid crystal molecules in a transverse electric field generated by the electrodes disposed on the substrate surface (JP 57-000618 A; JP 50-093665 A; JP 10-153782 A; JP 10-186351A; JP 10-333171A; JP 11-024068 A; JP 2008-020521A; Proc. 13th IDW, 97 (1997); Proc. 13th IDW, 175 (1997); SID Sym. Digest, 319 (1998); SID Sym. Digest, 838 (1998); SID Sym. Digest, 1085 (1998); SID Sym. Digest, 334 (2000); and Eurodisplay Proc., 142 (2009)). In this method, as an electric field in the transverse direction curves, liquid crystal molecules align in a different direction when a voltage is applied; therefore, multiple domains can be formed without performing pixel partitioning as in the case of the MVA mode described above. Accordingly, the method is excellent in view of production efficiency. Liquid crystal display devices of such a mode are called, according to JP 10-153782 A; JP 10-186351A; JP 10-333171A; JP 11-024068 A; JP 2008-020521A; Proc. 13th IDW, 97 (1997); Proc. 13th IDW, 175 (1997); SID Sym. Digest, 319 (1998); SID Sym. Digest, 838 (1998); SID Sym. Digest, 1085 (1998); SID Sym. Digest, 334 (2000); and Eurodisplay Proc., 142 (2009), by various names such as EOC and VA-IPS, but in the present invention, the display mode will be hereinafter abbreviated as “VAIPS”.
However, in the VAIPS mode, since the physical behavior of liquid crystal molecules is different from the conventional method for driving a liquid crystal display device, it is required to select a liquid crystal material under a criterion different from the conventional criteria in connection with the liquid crystal material.
That is, in general, the threshold voltage (Vc) of Fréedericksz transition in a twisted nematic (TN) mode is represented by the following formula:
                              Vc          =                                                    π                ⁢                                                                  ⁢                                  d                  cell                                                                              d                  cell                                +                                  〈                                      r                    ⁢                                                                                  ⁢                    1                                    〉                                                      ⁢                                                            K                  ⁢                                                                          ⁢                  11                                                  Δ                  ⁢                                                                          ⁢                  ɛ                                                                    ;                            [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
the same threshold voltage in a super-twisted nematic (STN) mode is represented by the following formula:
                              Vc          =                                                    π                ⁢                                                                  ⁢                                  d                  gap                                                                              d                  cell                                +                                  〈                                      r                    ⁢                                                                                  ⁢                    2                                    〉                                                      ⁢                                                            K                  ⁢                                                                          ⁢                  22                                                  Δ                  ⁢                                                                          ⁢                  ɛ                                                                    ;                            [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          2                ]            and
the same threshold voltage in the VA mode is represented by the following formula:
                    Vc        =                                            π              ⁢                                                          ⁢                              d                cell                                                                    d                cell                            -                              〈                                  r                  ⁢                                                                          ⁢                  3                                〉                                              ⁢                                                    K                ⁢                                                                  ⁢                33                                                                              Δ                  ⁢                                                                          ⁢                  ɛ                                                                                                        [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          3                ]            wherein Vc represents the Fréedericksz transition (V); π represents the ratio of the circumference of a circle to its diameter; dcell represents the distance (μm) between a first substrate and a second substrate; dgap represents the distance (μm) between a pixel electrode and a common electrode; dITO represents the width (μm) of the pixel electrode and/or common electrode; <r1>, <r2> and <r3> represent extrapolation lengths (μm); K11 represents the elastic constant (N) of splay; K22 represents the elastic constant (N) of twist; K33 represents the elastic constant (N) of bend; and Δ∈ represents dielectric anisotropy.
However, in the VAIPS mode, since these general calculation formulas do not fit, and no criteria for selecting the liquid crystal material are available, there has been no progress in the improvement of performance, and consequently, application thereof into liquid crystal display devices has been delayed.
On the other hand, in regard to the VAIPS mode, disclosures have also been made on preferred compounds as the liquid crystal material to be used (JP 2002-012867 A). However, the liquid crystal composition described in the relevant reference document uses a cyano-based compound, and therefore, the liquid crystal composition is not suitable for active matrix applications.
Liquid crystal display devices also have a problem of aiming to achieve mega contrast (CR) by enhancing the black level with a bright luminance. It has been suggested to improve the numerical aperture so as to enable increasing the pixel display area of LCDs, to apply a luminance enhancing film such as a dual brightness enhancement film (DBEF) or a cholesteric liquid crystal (CLC) film, or to reduce the light leakage caused by protrusions and the like when the liquid crystal is subjected to vertical alignment. Furthermore, there is also a demand for a display which is not easily brought into disorder even under a pressing pressure in a touch panel system.